The first step of our vision takes place in the retina where light is captured in the outer segment of photoreceptor cells. This light-sensing organelle is unique in that it is a modified primary cilium that is concentrated with proteins involved in visual signal transduction. Synthesis of these proteins occurs in the cell soma, followed by their delivery to the outer segment by discrete intracellular trafficking mechanisms. The visual pigment rhodopsin is the most abundant protein of the outer segment membranes and its targeting from the ER/Golgi to the outer segments is mediated by a small guanine nucleotide-binding protein Arf4. Though intracellular targeting and trafficking of rhodopsin is understood better than any other photoreceptor-specific protein, many mechanistic aspects remain unclear including the molecular event activating Arf4 to initiate rhodopsin sorting. As other guanine nucleotide-binding proteins, Arf4 is expected to be activated upon GDP/GTP exchange. Thus the first aim of my proposal is to determine the guanine nucleotide exchange factor that fulfills this function in photoreceptors. My second aim is to identify whether Arf4 is uniquely required for rhodopsin sorting and whether other proteins rely on Arf4 for targeting to the outer segments. I will also explore the hypothesis that Arf4 may be engaged in protein sorting to ciliary organelles in other cells. This project has broad implication for our general understanding of pathobiological processes underlying cases of inherited retinal degenerations, which are caused by mutations affecting protein sorting to the outer segment of photoreceptors.